Wear-resistant silicon nitride member and method of manufacture thereof

ABSTRACT

The present invention provides a wear resistant member composed of silicon nitride sintered body containing 2-10 mass % of rare earth element in terms of oxide thereof as sintering agent, 2-7 mass % of MgAl 2 O 4  spinel, 1-10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of said silicon nitride sintered body is 1 vol. % or less, a three-point bending strength is 900 MPa or more, and a fracture toughness is 6.3 MPa·m 1/2  or more. According to the above structure of the present invention, there can be provided a silicon nitride wear resistant member and a method of manufacturing the member having a high strength and a toughness property, and particularly excellent in rolling and sliding characteristics.

TECHNICAL FIELD

[0001] The present invention relates to a wear resistant member mainlycomposed of silicon nitride and a method of manufacturing the member,and more particularly to a silicon nitride wear resistant member and amethod of manufacturing the member capable of exhibiting excellent wearresistance, particularly rolling life characteristics when the wearresistant member is used as rolling bearing member, and is suitable as amaterial for constituting a rolling bearing member requiring anexcellent durability, and the wear resistant member also having a highdensity equal to or higher than the conventional silicon nitridesintered body and a high mechanical strength which is inherent to thesilicon nitride sintered body, even if the sintered body is manufacturedthrough a sintering operation under a low temperature of 1600° C. orlower.

BACKGROUND ART

[0002] Various sintering compositions for the silicon nitride sinteredbodies are well known: such as silicon nitride/oxide of rare earthelement/aluminum oxide system; silicon nitride/yttrium oxide/aluminumoxide/aluminum nitride system; and silicon nitride/oxide of rare earthelement/aluminum oxide/titanium oxide system or the like. Sinteringassistant agents composed of the oxides of rare earth elements, such asyttrium oxide (Y₂O₃) in the sintering compositions listed above, have afunction of generating grain boundary phase (liquid phase) composed ofSi-rare earth element-Al—O—N or the like during the sintering operation.Therefore, the sintering assistant agents are added to a materialcomposition for enhancing the sintering characteristics of sinteringmaterials, and achieve high density and high strength of the sinteredbodies.

[0003] According to the conventional art, the silicon nitride sinteredbodies are generally mass-produced as follows. After a sinteringassistant agent as mentioned above is added to the material powder ofsilicon nitride, the material mixture is molded to form a compact. Thusobtained compact is then sintered in a sintering furnace at a hightemperature of about 1,700-1,900° C. for a predetermined period of time.

[0004] However, in the conventional manufacturing method describedabove, since the sintering temperature was greatly high to be about 1700to 1900° C., there had been raised the following problems. That is, itwas required to upgrade a heat-resistant specification for the sinteringfurnace and ancillary equipments thereof, so that an installation costof the manufacturing facilities was greatly increased. Further, it wasdifficult to adopt a continuous manufacturing process, so that amanufacturing cost of the silicon nitride sintered body was remarkablyincreased and a mass-productivity of the sintered body wasdisadvantageously lowered.

[0005] In addition, although the silicon nitride sintered body producedby the conventional method achieves an improved bending strength,fracture toughness and wear resistance, however, the improvement isinsufficient. A durability as a rolling bearing member requiring aparticularly excellent sliding property is insufficient, so that afurther improvement has been demanded.

[0006] In these days, a demand of ceramic material as precision devicemembers has increased. In these applications, advantages such as highhardness and light weight together with high corrosion resistance andlow thermal expansion property of the ceramic are utilized. Inparticular, in view of the high-hardness and high wear resistance,application as a wear resistant member for constituting a slidingportion of the bearing or the like has been rapidly extended.

[0007] However, in a case where rolling balls of a bearing or the likewere constituted by the wear resistant member composed of ceramic, whenthe rolling balls were rolled while being repeatedly contacted withcounterpart at a high stress level, the rolling life of the wearresistant member was not sufficient yet. Therefore, a surface of thewear resistant member is peeled off and the member causes cracks, sothat the defective member was liable to causes vibration and damage to adevice equipped with the bearing. At any rate, there had been posed aproblem that the durability and reliability as a material forconstituting the parts of the device was low.

[0008] The present invention had been achieved for solving theaforementioned problems. Accordingly, an object of the present inventionis to provide a wear resistant member and a method of manufacturing themember excellent in wear resistance, particularly excellent in rollinglife characteristics that are suitable as rolling bearing member, inaddition to a high density equal to or higher than the conventionalsilicon nitride sintered body and a high mechanical strength which isinherent to the silicon nitride sintered body, even if the sintered bodyis manufactured through a sintering operation under a low temperature of1600° C. or lower.

DISCLOSURE OF THE INVENTION

[0009] In order to attain the objects described above, the inventors ofthe present invention had studied the effects and influences ofparameters such as the type of silicon nitride material powder,sintering assistant agent and additives, the amounts thereof, and thesintering conditions on the characteristics of the final products (i.e.,the sintered bodies) by performing experiments.

[0010] As a result, the experiments provided the following findings.That is, a sintering property was greatly improved, when certain amountsof a rare earth element, MgAl₂O₄ spinel or a mixture of magnesium oxideand aluminum oxide, silicon carbide, at least one element selected fromthe group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr, were added toa fine material powder of silicon nitride to prepare a material mixture.

[0011] Further, when the material mixture was molded and sintered at alow temperature of 1600° C. or lower; or when the sintered body aftercompletion of the sintering operation was further subjected to a hotisostatic pressing (HIP) treatment under predetermined conditions, awear resistant member composed of a silicon nitride sintered body havingan excellent wear resistance and a particularly excellent rolling lifeof sliding property could be obtained in addition to high density andhigh mechanical strength property that were equal to or higher thanthose of conventional silicon nitride sintered body.

[0012] The present invention had been achieved on the basis of theaforementioned findings.

[0013] That is, according to the present invention, there is provided awear resistant member composed of silicon nitride sintered bodycontaining 2-10 mass % of rare earth element in terms of oxide thereofas sintering agent, 2-7 mass % of MgAl₂O₄ spinel, 1-10 mass % of siliconcarbide, and 5 mass % or less of at least one element selected from thegroup consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxidethereof, wherein a porosity of said silicon nitride sintered body is 1vol. % or less, a three-point bending strength is 900 MPa or more, and afracture toughness is 6.3 MPa·m^(1/2) or more.

[0014] Further, the same function and effect can be obtained even in acase where a mixture of magnesium oxide and aluminum oxide is added asan additive component in place of MgAl₂O₄ spinel. Therefore, accordingto the present invention, there is provided another wear resistantmember composed of silicon nitride sintered body containing 2-10 mass %of rare earth element in terms of oxide thereof as sintering agent, 1-2mass % of magnesium oxide, 2-5 mass % of aluminum oxide, 1-10 mass % ofsilicon carbide, and 5 mass % or less of at least one element selectedfrom the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in termsof oxide thereof, wherein a porosity of said silicon nitride sinteredbody is 1 vol. % or less, a three-point bending strength is 900 MPa ormore, and a fracture toughness is 6.3 MPa·ml² or more.

[0015] Further, in the above wear resistant member composed of siliconnitride sintered body, it is preferable that a maximum width ofaggregated segregation existing in grain boundary phase of the siliconnitride sintered body is 5 μm or less.

[0016] Furthermore, it is preferable that an average width of theaggregated segregation existing in grain boundary phase of the siliconnitride sintered body is 2 μm or less.

[0017] When the silicon nitride material mixture is sintered, thesintering assistant agent and compounds of the additive components aretransformed into liquid phase thereby to form grain boundary phase. Whenthis liquid phase components in the grain boundary phase are aggregatedand segregated to become large, the mechanical strength of the sinteredbody is lowered. In particular, when the sintered body is used as thewear resistant member, the rolling property is disadvantageouslylowered. Therefore, it is preferable that the sintered body has a finestructure in which the maximum width of the aggregated segregationexisting in grain boundary phase is 5 μm or less, and the average widthof the aggregated segregation is 2 μm or less.

[0018] Further, the three point bending strength of the silicon nitridesintered body constituting above wear resistant member is 900 MPa ormore and a fracture toughness is 6.3 MPa·m^(1/2) or more. Therefore,there can be also formed a wear resistant member composed of the siliconnitride sintered body such that a rolling life defined as a rotationnumber of steel balls rolling along a circular track formed on the wearresistant member formed of the silicon nitride sintered body until asurface of the silicon nitride wear resistant member peels off is 1×10⁷or more, when the rolling life is measured in such a manner that acircular track having a diameter of 40 mm is set on the wear resistantmember, three rolling balls each having a diameter of 9.35 mm andcomposed of SUJ2 are provided on the circular track, and the rollingballs are rotated on the track at a rotation speed of 1200 rpm under acondition of being applied with a pressing load of 39.2 MPa.

[0019] Furthermore, in the above wear resistant member, it is preferablethat the silicon nitride sintered body has a crash strength of 200 MPaor more, and a rolling fatigue life defined as a time until a surface ofrolling balls composed of the silicon nitride wear resistant memberrolling along a circular track formed on a steel plate peels off is 400hours or more, when the rolling fatigue life is measured in such amanner that three rolling balls each having a diameter of 9.35 mm areformed from the silicon nitride wear resistant member, the three rollingballs are provided on the circular track having a diameter of 40 mm seton the steel plate formed of SUJ2, and the rolling balls are rotated ata rotation speed of 1200 rpm on the track under a condition of beingapplied with a pressing load so as to impart a maximum contact stress of5.9 GPa to the balls.

[0020] Further, in the wear resistant member according to the presentinvention, the silicon nitride sintered body contains at most 5 mass %of at least one element selected from the group consisting of Ti, Hf,Zr, W, Mo, Ta, Nb and Cr in terms of oxide thereof.

[0021] Further, when the wear resistant member composed of above thesilicon nitride sintered body is a rolling bearing member, the wearresistant member can exhibit particularly excellent sliding property anddurability.

[0022] Further, there is provided a method of manufacturing a wearresistant member composed of silicon nitride sintered body comprisingthe steps of: preparing a material mixture by adding 2-10 mass % of arare earth element in terms of the amount of an oxide thereof, 2-7 mass% of MgAl₂O₄ spinel, 1-10 mass % of silicon carbide, and 5 mass % orless of at least one element selected from the group consisting of Ti,Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, to a siliconnitride powder containing 1.5 mass % or less of oxygen and 90 mass % ormore of a -phase type silicon nitride and having an average grain sizeof 1 μm or less; molding the material mixture to form a compact; andsintering the compact in non-oxidizing atmosphere at a temperature of1,600° C. or lower thereby to form a wear resistant member composed ofsilicon nitride sintered body.

[0023] In the above manufacturing method, the same function and effectcan be obtained even in a case where a mixture of magnesium oxide andaluminum oxide is added as an additive component in place of MgAl₂O₄spinel. Therefore, according to the present invention, there is providedanother method of manufacturing a wear resistant member composed ofsilicon nitride sintered body comprising the steps of: preparing amaterial mixture by adding 2-10 mass % of a rare earth element in termsof the amount of an oxide thereof, 1-2 mass % of magnesium oxide, 2-5mass % of aluminum oxide, 1-10 mass % of silicon carbide, and 5 mass %or less of at least one element selected from the group consisting ofTi, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, to a siliconnitride powder containing 1.5 mass % or less of oxygen and 90 mass % ormore of α-phase type silicon nitride and having an average grain size of1 μm or less; molding the material mixture to form a compact; andsintering the compact in non-oxidizing atmosphere at a temperature of1,600° C. or lower thereby to form a wear resistant member composed ofsilicon nitride sintered body.

[0024] In the above method of manufacturing the wear resistant member,it is preferable that the method further comprises a step of: conductinga hot isostatic pressing (HIP) treatment to the silicon nitride sinteredbody in non-oxidizing atmosphere of 30 MPa or more at a temperature of1,600° C. or lower after completion of the sintering step.

[0025] According to the above manufacturing method, the oxide of rareearth element, MgAl₂O₄ spinel or the mixture of magnesium oxide andaluminum oxide, silicon carbide, and compound of Ti, Zr, Hf or the likeare added to the silicon nitride material powder when the siliconnitride sintered body constituting the wear resistant member isprepared. Therefore, MgAl₂O₄ spinel together with the oxide of rareearth element such as yttrium oxide or the like react with siliconnitride material powder to generate the liquid phase having a lowmelting point and function as sintering promoting agent, so that adensification of the molded body can be advanced at low temperature of1600° C. or lower, and the sintering assistant agent exhibits a functionof suppressing grain growth in the crystal structure whereby thestructure of the sintered body is made fine and the mechanical strengthis improved.

[0026] Further, silicon carbide (SiC) is solely dispersed as particlesin a structure of the sintered body, and has a function of significantlyimproving the rolling fatigue characteristics of the silicon nitridesintered body. On the other hand, the compound of Ti, Zr, Hf or the likepromotes the function as the sintering assistant agent of rare earthelement oxide or the like, and the compound also has a function ofdispersion-reinforcing the crystal structure as the same manner as SiC,thereby to improve the mechanical strength of the sintered body. As aresult, there can be obtained a wear resistant member composed ofsilicon nitride sintered body having a fine structure and excellence inmechanical properties such that a maximum width of aggregatedsegregation existing in grain boundary phase is 5 μm or less and anaverage width of aggregated segregation is 2 μm or less, the maximumsize of the pores is 0.4 μm or less, porosity is 1 vol. % or less, threepoint bending strength at room temperature is 900 MPa or more, fracturetoughness of 6.3 MPa·m^(1/2) or more and crush strength is 200 MPa ormore, and having excellent mechanical properties.

[0027] To achieve good sintering characteristic, high bending strength,high fracture toughness value and long rolling life of the product, thesilicon nitride fine powder which is used in the method of the inventionand contained as a main component in the sintered body constituting thewear resistant member of the invention preferably contains at most 1.7mass %, preferably, 0.7-1.5 mass % of oxygen and 90 mass % or more, morepreferably, 92-97 mass % of alpha-phase type silicon nitride, andfurther the powder has fine grains, that is, an average grain size of atmost 1 μm, more preferably about 0.4-0.8 μm.

[0028] By the way, as the silicon nitride material powder, two types ofα-phase type Si₃N₄ powder and β-phase type Si₃N₄ powder have been known.However, when a sintered body is formed from the α-phase type Si₃N₄powder, there is a tendency that a strength is liable to beinsufficient. In contrast, in case of the β-phase type Si₃N₄ powder,although a high temperature is required for the sintering operation,there can be obtained a sintered body having a high strength and astructure in which a number of silicon nitride fibers each having alarge aspect ratio are tangled in a complicate manner.

[0029] In this connection, in the present invention, since the sinteredbody is prepared by sintering α-phase type Si₃N₄ material powder at alow temperature of 1600° C. or lower, there can be obtained a sinteredbody in which α-phase type Si₃N₄ crystal grains and β-phase type Si₃N₄crystal grains are coexisting in the crystal structure. Accordingly,since a small amount of α-phase type Si₃N₄ crystal grains coexists amongthe β-phase type Si₃N₄ crystal grains, a substantial structure of acomposite material is formed, so that the strength and toughness valueof the sintered body can be improved.

[0030] In this invention, a reason why a blending amount of α-phase typeSi₃N₄ powder is limited to a range of 90 mass % (wt %) or more is asfollows. That is, when the amount is set to a range of 90 mass % ormore, a bending strength, fracture toughness and rolling life of theSi₃N₄ sintered body are greatly increased thereby to further improve theexcellent characteristics of the silicon nitride. On the other hand, theamount is limited to at most 97 mass % in view of the sinteringproperty. It is more preferable to set the range to 92-95 mass %.

[0031] As a result, in order to achieve a good sintering characteristic,high bending strength, high fracture toughness and long rolling life ofthe product, as a starting material powder of the silicon nitride, it ispreferable to use the silicon nitride fine powder containing at most 1.7mass %, preferably, 0.7-1.5 mass % of oxygen, and at least 90 mass % ofalpha-phase type silicon nitride, and further the powder has finegrains, that is, an average-grain size of at most 1 g m, more preferablyabout 0.4-0.8 μm.

[0032] In particular, the use of a fine powder of silicon nitride havingan average grain size of 0.7 μm or less facilitates forming a densesintered body having a porosity of at most 1% by volume withoutrequiring a large amount of a sintering assistant agent. The porosity ofthe sintered body can be easily measured in accordance with aArchimedes' method.

[0033] A total oxygen content contained in the silicon nitride sinteredbody constituting the wear resistant member of the present invention isspecified to 4.5 mass % or less. When the total oxygen content in thesintered body exceeds 4.5 mass %, a maximum size of the pore formed inthe grain boundary phase is disadvantageously increased, and the pore isliable to be a starting point of a fatigue failure, thereby to lower therolling (fatigue) life of the wear resistant member. A preferable rangeof the total oxygen content is 4 mass % or less.

[0034] By the way, the term “total oxygen content of the sintered body”specified in the present invention denotes a total amount in terms ofmass % of oxygen constituting the silicon nitride sintered body.Accordingly, when the oxygen exists in the silicon nitride sintered bodyas compounds such as metal oxide, oxidized nitride or the like, thetotal oxygen content is not an amount of the metal oxide (and oxidizednitride) but an amount of oxygen in the metal oxide (and the oxidizednitride).

[0035] The maximum pore size formed in the grain boundaries of thesilicon nitride sintered body constituting the wear resistant member ofthe present invention is preferably specified to 0.4 μm or less. Whenthe maximum pore size exceeds 0.4 μm, the pore is liable to particularlybe a starting point of a fatigue failure, thereby to lower the rolling(fatigue) life of the wear resistant member. A preferable range of themaximum pore size (diameter) is 0.2 μm or less.

[0036] Examples of the rare earth element to be added as a sinteringassistant agent to a silicon nitride powder are Y, Ho, Er, Yb, La, Sc,Pr, Ce, Nd, Dy, Sm and Gd. Such a rare earth element may be added to thesilicon nitride powder in the form of an oxide thereof or a substancewhich is changed into an oxide thereof during the sintering process. Twoor more kinds of such oxide or substance may be added to the siliconnitride powder in a combination manner. Such a sintering assistant agentreacts with the silicon nitride powder so as to form a liquid phase andthereby serves as a sintering promoter.

[0037] The amount of a sintering assistant agent to be added to thematerial powder is set to be within a range of from 2 to 10 mass % interms of the amount of an oxide thereof. If the amount is less than 2mass %, the sintered body fails to achieve a sufficiently high densityand high strength. In particular, when an element which has a largeatomic weight like lanthanoid is used as the rare earth element at aboveless amount, a sintered body having a relatively low strength andrelatively low thermal conductivity is formed.

[0038] On the other hand, if the amount is more than 10 mass %, anexcessively large portion of the grain boundary phase is formed, and thegeneration of pore is increased, thereby reducing the strength of thesintered body. For this reason, the amount of a sintering assistantagent is within the range described above. For the same reason describedabove, the more preferred range of the amount of a sintering assistantagent is 3 to 8 mass %.

[0039] In the present invention, MgAl₂O₄ spinel together with rare earthelement oxide such as yttrium oxide or the like to be used as additioncomponents react with silicon nitride material powder to generate theliquid phase having a low melting point and function as sinteringpromoting agent, so that a densification of the sintered body at lowtemperature of 1600° C. or lower can be performed, and MgAl₂O₄ spinelexhibits a function of controlling and suppressing grain growth in thecrystal structure whereby the structure of Si₃N₄ sintered body is madefine and the mechanical strength is improved. Further, MgAl₂O₄ spinelfunctions to lower a transition temperature at which α-phase typesilicon nitride is transformed into β-phase type silicon nitride, sothat the densification is advanced at a low temperature. Therefore,α-phase type silicon nitride phase should be remained to some extent inthe crystal structure after the sintering operation whereby to increasestrength and fracture toughness value of the resultant sintered body.

[0040] The same function and effect can be obtained even in a case wherea mixture of magnesium oxide (MgO) and aluminum oxide (Al₂O₃) is addedas additive components in place of MgAl₂O₄ spinel. In this case, anaddition amount of MgO is specified to a range of 1-2 mass %. If theaddition amount of MgO is less than 1 mass %, the densification of thesintered body becomes insufficient. On the other hand, if the amount isexcessively large so as to exceed 2 mass %, the strength of the sinteredbody and the rolling fatigue characteristic as a wear resistant memberare disadvantageously lowered.

[0041] Further, an addition amount of Al₂O₃ is specified to a range of2-5 mass %. If the addition amount of Al₂O₃ is less than 2 mass %, thedensification of the sintered body becomes insufficient. On the otherhand, if the amount is excessively large so as to exceed 5 mass %, thestrength of the sintered body and the rolling fatigue characteristic asa wear resistant member are disadvantageously lowered.

[0042] Furthermore, silicon carbide (SiC) to be used as another additioncomponent in the present invention is added within a range of 1-10 mass% for the purpose of being solely dispersed as particles in the crystalstructure, and for exhibiting a function of drastically improving therolling life characteristic of the silicon nitride sintered body. Inaddition, silicon carbide (SiC) is added for improving the mechanicalstrength such as bending strength and fracture toughness value or thelike of the Si₃N₄ sintered body.

[0043] When the addition amount of silicon carbide (SiC) is less than 1mass %, the sintered body fails to achieve a sufficiently additioneffect. On the other hand, when the addition amount is excessively largeto exceed 10 mass %, the densification of the sintered body becomesinsufficient thereby to lower the bending strength of the sintered body.For this reason, the addition amount of SiC is set to the range of 1-10mass %, preferably to a range of 3-7 mass %. In particular, in order tosecure good performances together with sintering property, strength androlling life, it is preferable to set the addition amount of SiC to arange of 3.5-6 mass %.

[0044] In this regard, there exist two types of silicon carbides i.e.,α-type and β-type silicon carbides. However, both α-type and β-typesilicon carbides exhibit the same function and effect to each other.

[0045] Further, in the present invention, at least one element selectedfrom the group consisting of Ti, Hf, Zr, W, Mo, Ta, Nb and Cr is alsoadded as another component at an amount of 5 mass % or less. Theseelements to be used as another addition component are added to thesilicon nitride material powder as oxides, carbides, nitrides, silicidesand borides thereof. These compounds promote the sintering assistanteffect of the rare earth element, and also function to further lower thetransition temperature at which α-phase type silicon nitride istransformed into β-phase type silicon nitride, and also promotedispersion thereof in the crystal structure so as to enhance themechanical strength of the silicon nitride (Si₃N₄) sintered body. Amongthem, compounds of Ti, Zr and Hf are particularly preferred.

[0046] If the addition amount of these compounds is less than 0.3 mass%, the sintered body fails to achieve a sufficiently addition effect. Onthe other hand, if the amount is excessively large so as to exceed 5mass %, the mechanical strength and rolling life of the sintered bodyare disadvantageously lowered. For this reason, the preferred range ofthe amount of these compounds contained is at most 5 mass %. Inparticular, the amount is more preferably set to a range of 0.5-3 mass%.

[0047] The above compounds such as Ti, Zr, Hf or the like also serve aslight blocking agents (light shielding agents). More specifically, theycolor the silicon nitride type ceramic sintered body black and thusprovides it with an opacity.

[0048] Further, since the porosity of the sintered body has a greatinfluence on the rolling life and bending strength of the wear resistantmember, so that the sintered body should be manufactured so as toprovide the porosity of 1 vol. % or less. If the porosity exceeds 1% byvolume, the pore to be a starting point of the fatigue failure israpidly increased, thereby to lower the strength of the sintered bodyand shorten the rolling life of the wear resistant member.

[0049] The silicon nitride sintered body constituting the wear resistantmember according to the present invention can be produced by, forexample, the following processes. A material mixture is prepared byadding predetermined amount of a sintering assistant agent, MgAl₂O₄spinel or the mixture of magnesium oxide and aluminum oxide, siliconcarbide, a required additive such as an organic binder, and a compoundof Ti or the like, to a fine powder of silicon nitride which has apredetermined fine average grain size and contains very small amount ofoxygen. The material mixture is then molded into a compact having apredetermined shape. As a method of molding the material mixture,conventional molding methods such as the die-pressing method or thedoctor-blade method, rubber-pressing method, CIP (cold isostaticpressing) method or the like can be applied.

[0050] In a case where the molded compact is prepared through the abovedie-press-molding method, in order to particularly form a grain boundaryhardly causing the pores or voids, it is preferable to set the moldingpressure for the material mixture to 120 MPa or more. When the moldingpressure is less than 120 MPa, there are easily formed portions(segregated portions) to which the compound of rare earth element ascomponent mainly constituting the grain boundary is agglomerated, andthe compact cannot be sufficiently densified, so that there is obtaineda sintered body with many crack-formations.

[0051] Further, the above agglomerated portion (segregated portion) inthe grain boundary is liable to become a starting point of fatiguefailure, thus lowering the life and durability of the wear resistantmember. On the other hand, when the molding pressure is set to anexcessively large value so as to exceed 200 MPa, a durability of themolding die is disadvantageously lowered, and it cannot be always saidthat the productivity is good. Therefore, the above molding pressure ispreferably set to a range of 120-200 MPa.

[0052] Subsequent to the above molding process, the molded compact isheated and maintained at 600-800° C. for 1-2 hours in a non-oxidizingatmosphere or at 400-500° C. for 1-2 hours in the air, therebydegreasing the compact, that is, thoroughly removing the organic bindercomponent added in the material mixture preparing process.

[0053] The degreased compact is then sintered bynormal-pressure-sintering method or pressured-sintering method at atemperature of 1,600° C. or lower for 0.5-10 hours in non-oxidizingatmosphere filled with inert gas such as argon gas or nitrogen gas orhydrogen gas. As the pressured-sintering method, various press-sinteringmethods such as a pressurized-atmosphere sintering method, hot-pressingmethod, HIP (hot isostatic pressing) method or the like can be utilized.

[0054] In addition, when the silicon nitride sintered body is furthersubjected to a hot isostatic pressing (HIP) treatment under atemperature condition of 1,600° C. or lower in non-oxidizing atmosphereof 30 MPa or lower, an influence of the pore constituting a startingpoint of fatigue failure of the sintered body can be further reduced, sothat there can be obtained a wear resistant member having a furtherimproved sliding property and rolling life characteristics.

[0055] The silicon nitride wear resistant member produced by the abovemethod achieves a total oxygen content of 4.5 mass % or less, a porosityof 1% or less, a maximum pore size (diameter) of 0.4 μm or less andexcellent mechanical characteristics, that is, a three-point bendingstrength (at room temperature) of 900 MPa or greater.

[0056] Further, there can be also obtained a silicon nitride wearresistant member having a crush strength of 200 MPa or more and afracture toughness of 6.3 MPa·m^(1/2) or more.

[0057] According to the silicon nitride wear resistant member and themethod of manufacturing the member of the present invention, thematerial mixture is prepared by adding the predetermined amounts of therare earth element, MgAl₂O₄ spinel or the mixture of magnesium oxide andaluminum oxide, silicon carbide, and compound of Ti, Zr, Hf or the liketo the silicon nitride material powder, so that the sintering propertyis greatly improved. Therefore, even if the molded compact is sinteredat a low temperature of 1600° C. or lower, there can be obtained asilicon nitride wear resistant member having an excellent wearresistance, a high density and a high mechanical strength that are equalto or higher than those of conventional silicon nitride sintered body.In particular, the silicon nitride wear resistant member is suitable fora material constituting a rolling bearing member in view of itsexcellent rolling life characteristics.

[0058] In other word, according to the wear resistant member of thepresent invention, the grain growth of the silicon nitride crystalgrains can be suppressed by using the predetermined sintering assistantagent and by setting the sintering temperature to 1600° C. or lower.Since the grain growth can be effectively suppressed, a triple pointformed among the silicon nitride crystal grains can be minimized, sothat it becomes possible to make a width of the grain boundary phasesmall.

[0059] Further, since the sintering temperature is set to a lower levelof 1600° C. or lower, the width of the grain boundary phase formedduring the sintering process can be decreased. Simultaneously, anevaporation of the components of the grain boundary phase or impuritiescontained in the grain boundary phase are prevented from beingdischarged to outside. Therefore, the generation of pores is suppressedand a maximum size (diameter) of pore can be minimized, so that therecan be obtained a wear resistant member excellent in rolling lifecharacteristics and durability. Accordingly, when a bearing device isprepared by using this wear resistant member as rolling bearing member,good sliding/rolling characteristics can be maintained for a long timeof period, and there can be provided a rotation machine having excellentoperational reliability and durability. Further, as an example ofanother application, the wear resistant member of this invention can beapplied to various fields such as engine parts, various tool material,various rails, various rollers or the like which require an excellentwear resistance.

BRIEF DESCRIPTION OF THE DRAWING

[0060]FIG. 1 is a cross sectional view showing a thrust-type rollingabrasion (wear) testing machine for measuring rolling lifecharacteristics of a wear resistant member according to one embodimentof the present invention. Explanation of the Reference Numerals1—machine body, 2—wear resistant member, 3—rolling steel ball, 4—guideplate, 5—driving rotation shaft, 6—retainer, 7—lubrication oil,8—rolling ball (made of silicon nitride), 9—bearing steel plate (SUJ2plate)

BEST MODE FOR EMBODYING THE INVENTION

[0061] Next, preferred embodiments of the silicon nitride wear resistantmember according to the present invention will be explained moreconcretely on the basis of the following Examples and ComparativeExamples.

EXAMPLES 1-3

[0062] A material powder mixture for Examples 1 was prepared by adding 5mass % of Y₂O₃ (yttrium oxide) powder having an average grain size of0.9 μm, 5 mass % of MgAl₂O₄ spinel powder having an average grain sizeof 0.5 μm, 5 mass % of β-phase type SiC (silicon carbide) powder havingan average grain size of 0.8 μm, and 1 mass % of ZrO₂ (zirconium oxide)powder having an average grain size of 0.6 μm, as sintering assistantagents, to 86 mass % of Si₃N₄ (silicon nitride) material powdercontaining 1.3 mass % of oxygen, and 97% of α-phase type siliconnitride, and having an average grain size of 0.55 μm, followed bywet-mixing the materials in ethyl alcohol for 96 hours using pulverizingballs as pulverization media formed of silicon nitride, and drying themixture, thereby to prepare a material powder mixture.

[0063] After adding a predetermined amount of an organic binder and asolvent to the material powder mixture, thereby to prepare a blendedgranulated powder. Then, the granulated powder was press-molded at amolding pressure of 130 MPa, thereby to prepare a number of moldedcompacts each having a dimension of 50 mm (length)×50 mm (width)×5 mm(thickness) as samples for measuring bending strength and a number ofmolded compacts each having a dimension of 80 mm (diameter)×6 mm(thickness) as samples for measuring rolling life.

[0064] Thereafter, thus prepared molded compacts were degreased inair-flowing atmosphere at temperature of 450° C. for 4 hours.Thereafter, the degreased molded compacts were sintered and densified byholding the compacts in a nitrogen gas (N₂) atmosphere under a pressureof 0.7 MPa at a temperature of 1550° C. for 6 hours thereby to prepare anumber of silicon nitride wear resistant members of Example 1.

[0065] In addition, as Example 2, the manufacturing steps were repeatedunder the same conditions as in Example 1 except that a hot isostaticpressing (HIP) treatment was performed to the sintered body obtained inExample 1 in such a manner that the sintered body was heated andsintered in nitrogen gas atmosphere of 100 MPa at a temperature of 1500°C. for 6 hours, thereby to prepare a silicon nitride wear resistantmember of Example 2.

[0066] Further, as Example 3, the manufacturing steps were repeatedunder the same conditions as in Example 2 except that 1.5 mass % of MgO(magnesium oxide) powder having an average grain size of 0.5 μm and 3.5mass % of Al₂O₃ (aluminum oxide) having an average grain size of 0.8 μmwere added in place of MgAl₂O₄ spinel powder, thereby to prepare asilicon nitride wear resistant member of Example 3.

COMPARATIVE EXAMPLES 1-4

[0067] As Comparative Example 1, the manufacturing steps were repeatedunder the same conditions as in Example 1 except that the SiC powder wasnot added to the material mixture thereby to prepare silicon nitridewear resistant member of Comparative Example 1.

[0068] Further, as Comparative Example 2, a hot isostatic pressing (HIP)treatment was performed to the sintered body obtained in ComparativeExample 1 in such a manner that the sintered body was heated andsintered in nitrogen gas atmosphere of 100 MPa at a temperature of 1500°C. for 1 hour, thereby to prepare silicon nitride wear resistant memberof Comparative Example 2.

[0069] Furthermore, as Comparative Example-3, the manufacturing stepswere repeated under the same conditions as in Example 1 except that 5mass % of Al₂O₃ (aluminum oxide) powder having an average grain size of0.8 μm was added in place of MgAl₂O₄ spinel powder, thereby to preparesilicon nitride wear resistant member of Comparative Example 3.

[0070] Still further, as Comparative Example 4, the manufacturing stepswere repeated under the same conditions as in Example 2 except that theSi₃N₄ (silicon nitride) material powder containing 1.7 mass % of oxygenand 91 mass % of α-phase type silicon nitride, and having an averagegrain size of 1.5 μm was used, thereby to prepare silicon nitride wearresistant member of Comparative Example 4.

[0071] With respect to thus prepared silicon nitride wear resistantmembers of Examples and Comparative Examples, porosity, maximum widthand average width of the agglomerated segregations in the grainboundary, three-point bending strength at room temperature, fracturetoughness and rolling life were measured. The fracture toughness wasmeasured by Niihara system based on a micro-indentation method. Themeasured results are shown in table 1.

[0072] In addition, the porosity of the sintered body was measured byArchimedes' method, while the maximum width and average width of theaggregated segregations in the grain boundary phase was measured asfollows. Namely, three regions each having a unit area of 100μm-length×100 μm-width were arbitrarily set on a cross section of thesintered body constituting the wear resistant member, then an enlargedphotographic image (magnification of about 5000) was taken with respectto the regions by means of a scanning-type electron microscope (SEM).Among the aggregated segregations shown in the image, a segregationhaving the largest diameter was selected as a maximum size of theaggregated segregation. Concretely, the maximum width of the aggregatedsegregation was measured as a diameter of a minimum circlecircumscribing the triple-point region formed among the crystal grains.

[0073] Further, the average width of the aggregated segregations in thesilicon nitride sintered body was calculated as an average value of thesegregation widths at 20 sites in the observation field.

[0074] In this regard, when the structure of the silicon nitridesintered body is observed through a magnified photograph taken by SEM orthe like, the aggregated segregation is revealed and observed with ahighlighted color brighter than that of ordinary grain boundary phase.For example, in case of a monochrome photograph, the silicon nitridecrystal grains are revealed with a blackish color, while the grainboundary phase is revealed with a white color, and the aggregatedsegregation is revealed with a highlighted white color. Therefore, theaggregated segregation can be sharply and easily distinguished from thegrain boundary phase. Further, if necessary, when an existence of rareearth element is confirmed by EPMA, a concentration of the rare earthelement is revealed with a color darker than that of ordinary grainboundary phase, so that the respective constitutional elements can bedistinguished from one to other by means of this analyzing method.

[0075] Furthermore, the three-point bending strength was measured asfollows. That is, bending test pieces each having a dimension of 40 mm(length)×3 mm (width)×4 mm (thickness) were prepared from the respectivesintered bodies. Then, the test piece was supported at a supporting spanof 30 mm, while a load-applying speed was set to 0.5 mm/min. Under theseconditions, the three-point bending strength was measured.

[0076] Further, the rolling characteristics of the respective wearresistant members were measured by using a thrust-type rolling abrasiontesting machine shown in FIG. 1. This testing machine is constituted bycomprising: a plate-shaped wear resistant member 2 disposed in a machinebody 1; a plurality of rolling steel balls 3 provided on an uppersurface of the wear resistant member 2; a guide plate 4 provided at anupper portion of these rolling steel balls 3; a driving rotation shaft 5connected to the guide plate 4; and a retainer 6 for regulating alocation interval of the rolling steel balls 3. A lubricating oil 7 forlubricating a rolling portion of the balls is poured into the machinebody 1. The above rolling steel balls 3 and the guide plate 4 are formedof high-carbon-chromium bearing steel (SUJ2) prescribed by JIS G 4805(Japanese Industrial Standard). As the above lubricating oil 7, paraffintype lubricating oil (viscosity at 40° C.: 67.2 mm²/S) or turbine oilcan be used.

[0077] The rolling life of the respective plate-shaped wear resistantmembers of these embodiments were measured in such a manner that acircular track having a diameter of 40 mm was set on an upper surface ofthe wear resistant member 2, three rolling steel balls each having adiameter of 9.35 mm and composed of SUJ2 were provided on the circulartrack, and the rolling steel balls were rotated on the track at arotation speed of 1200 rpm under a condition of being applied with apressing load of 439.2 MPa and a condition of lubrication by an oil bathfilled with turbine oil thereby to measure the rolling life defined as arotation number of steel balls rolling along the circular track locatedon the wear resistant member formed of the silicon nitride sintered bodyuntil a surface of the silicon nitride wear resistant member 2 peeledoff. The measuring results are shown in Table 1 hereunder. TABLE 1 Widthof Aggregated Segregation in Liquid Three-Point Phase(Grain BoundaryBending Fracture Porosity Phase) (μm) Strength Toughness Rolling LifeSample (%) Average Maximum (MPa) (MPa · m^(1/2)) (rotations) Example 10.2 0.5 1 990 6.6   5 × 10⁷ Example 2 0.02 0.6 1.5 1100 6.9 >1 × 10⁸Example 3 0.02 0.6 1.5 1080 6.9 >1 × 10⁸ Comparative 0.2 3 6 900 6.1   2× 10⁶ Example 1 Comparative 0.02 3.5 6.5 1020 6.2   6 × 10⁶ Example 2Comparative 3.2 2.5 5.5 800 5.8   4 × 10⁵ Example 3 Comparative 1.3 3 6875 6.0   1 × 10⁶ Example 4

[0078] As is clear from the results shown in Table 1, in the respectivesilicon nitride wear resistant members of Examples, each of the sinteredbodies constituting the wear resistant members was manufactured so as tocontain a predetermined amount of additive components, so that thegeneration of pores or voids was effectively suppressed and the maximumwidth of the aggregated segregation was decreased to be fine wherebythere could be obtained silicon nitride wear resistant members havinggood strength characteristics and excellent rolling life and durability.Further, although not shown in Table 1, the maximum size (diameter) ofpores formed in the grain boundary phases of the respective wearresistant members according to all Examples was 0.4 μm or less.

[0079] On the other hand, in Comparative Example 1 in which the SiCcomponent was not contained, the amount of aggregated segregation of theliquid phase components was increased, thereby to disadvantageouslylower the strength characteristics and rolling life.

[0080] Further, in a case where the HIP treatment was performed and theSiC component was not contained as in Comparative Example 2, althoughthe three-point bending strength was high, the effect of decreasing theaggregated segregation was insufficient thereby to shorten the rollinglife of the wear resistant member.

[0081] Furthermore, in case of Comparative Example 3 where only Al₂O₃(aluminum oxide) powder was added in place of MgAl₂O₄ spinel powder, theporosity was disadvantageously large even if the sintering operation wassufficiently performed, and the width of the aggregated segregationbecame large, so that it was confirmed that both the strength androlling life were lowered.

[0082] Furthermore, in case of Comparative Example 4 where the oxygencontent in the silicon nitride material powder was excessively large,the amount of pores generated due to the high oxygen content was largeand the width of the aggregated segregation was increased, so that itwas confirmed that both the bending strength and rolling life werelowered.

[0083] Next, preferred embodiments of the silicon nitride wear resistantmember according to the present invention applied to rolling balls of abearing member will be explained more concretely on the basis of thefollowing Examples and Comparative Examples.

EXAMPLES 1B-3B and COMPARATIVE EXAMPLES 1B-4B

[0084] Each of the blended granulated powders as prepared in Examples1-3 and Comparative Examples 1-4 was packed in the molding die andpressed thereby to prepare spherical primary molded bodies. Then, eachof the primary molded bodies was subjected to a rubber pressingtreatment at a pressure of 100 MPa, thereby to respectively preparespherically molded bodies as samples each having a diameter of 11 mm formeasuring crush strength and rolling fatigue life.

[0085] Next, after the respective spherically molded bodies weresubjected to the degreasing treatment and sintering operation under thesame conditions as in the corresponding Examples and ComparativeExamples, there by to prepare the densified sintered bodies ofcorresponding Examples and Comparative Examples. Further, thus obtainedsintered bodies were subjected to grinding work so as to provide aball-shape having a diameter of 9.52 mm and a surface roughness of 0.01μm-Ra thereby to prepare the respective rolling balls for a bearing aswear resistant members of Examples 1B-3B and Comparative Examples 1B4B.

[0086] In this connection, the above surface roughness was measured asan arithmetic average surface roughness (Ra) which can be obtained byscanning the surface on equator of the ball by means of aprofilometer-type surface roughness measuring device.

[0087] With respect to thus prepared rolling balls as the wear resistantmembers of Examples and Comparative Examples, porosity, maximum andaverage widths of aggregated segregations in the grain boundary phase,crush strength at room temperature (25° C.), fracture toughness valueand rolling fatigue life were measured.

[0088] In this connection, the rolling fatigue life of the respectivewear resistant members were measured by using the thrust-type rollingabrasion testing machine shown in FIG. 1. By the way, in the previousExample 1 or the like, an item to be evaluated was a plate-shaped wearresistant member 2 while the balls rolling on the surface of the wearresistant member 2 were the rolling steel balls 3 composed of SUJ2.However, in order to evaluate the silicon nitride rolling balls 8 ofExamples 1B-3B and Comparative Examples 1B-4B, a bearing steel plate 9composed of SUJ2 was provided and assembled in place of the wearresistant member 2.

[0089] The rolling fatigue life of the respective rolling ball wasmeasured in such a manner that three rolling balls 8 each having adiameter of 9.52 mm were formed from the silicon nitride wear resistantmember, the three rolling balls 8 were provided on the circular trackhaving a diameter of 40 mm set on the upper surface of the steel plate 9formed of SUJ2, and the rolling balls 8 were rotated at a rotation speedof 1200 rpm on the track under a condition of being applied with apressing load so as to impart a maximum contact stress of 5.9 GPa to theballs 8 and a lubricating condition using an oil bath filled withturbine oil, thereby to measure the rolling fatigue life defined as atime until a surface of the rolling balls 8 composed of the siliconnitride wear resistant member peeled off. The measured results are shownin Table 2 hereunder. TABLE 2 Width of Aggregated Segregation in LiquidPhase(Grain Boundary Crush Fracture Rolling Phase) (μm) StrengthToughness Fatigue Life Sample Porosity (%) Average Maximum (MPa) (MPa ·m^(1/2)) (hr) Example 1B 0.2 0.5 1 220 6.6 >400 Example 2B 0.02 0.5 1.5270 6.8 >400 Example 3B 0.02 0.7 1.5 260 6.9 >400 Comparative 0.2 3 6200 6.1 250 Example 1B Comparative 0.02 3.5 6.5 240 6.2 300 Example 2BComparative 3.4 2.5 5.5 185 5.8 100 Example 3B Comparative 1.3 3 6 1906.0 200 Example 4B

[0090] As is clear from the results shown in Table 2, in the respectivesilicon nitride rolling balls of Examples, each of the sintered bodiesconstituting the rolling balls was manufactured so as to containpredetermined additive components, so that the generation of the poreswas effectively suppressed thereby to decrease the width of aggregatedsegregations in the grain boundary to be small. Therefore, there couldbe obtained the silicon nitride rolling balls each having a high crushstrength and an excellent durability such that the rolling fatigue lifeexceeds 400 hours.

[0091] On the other hand, in Comparative Example 1B in which SiC was notcontained in the balls, a large amount of pores remained in the sinteredbody thereby to disadvantageously lower the crush strength and therolling fatigue life. 115 Further, in a case where the HIP treatment wasperformed and the SiC component was not contained as in ComparativeExample 2B, the effect of minimizing the pore size was observed.However, the rolling fatigue life of the wear resistant member wasshortened.

[0092] Furthermore, in case of Comparative Example 3B where only Al₂O₃was contained in place of MgAl₂O₄ spinel, the porosity wasdisadvantageously increased even if the sintering operation wassufficiently performed, so that it was confirmed that both the crushstrength and rolling fatigue life of the wear resistant member werelowered.

[0093] Furthermore, in case of Comparative Example 4B where the siliconnitride material powder having an excessively large content of oxygenwas used, the amounts of liquid phase component and pores generated dueto the high oxygen content were large, so that it was confirmed that anyof the porosity, crush strength, fracture toughness value, and rollingfatigue life was insufficient.

[0094] In this connection, when the rolling fatigue life of the siliconnitride rolling balls were measured, three rolling balls each having adiameter of 9.52 mm were used. However, even if other balls havingdifferent diameters were selected or the number of balls to be providedwas changed, it was also confirmed that rolling properties in accordancewith the load conditions or the rolling conditions could be obtained.

[0095] Next, with respect to a plate-shaped wear resistant memberprepared through other compositions or treating conditions than those ofthe previous Examples will be explained more concretely with referenceto the following Examples and Comparative Examples.

EXAMPLES 4-35

[0096] Material mixtures for Examples 4-35 were prepared so as toprovide composition ratios shown in Tables 34 by blending Y₂O₃ powder,MgAl₂O₄ spinel powder, SiC powder used in Example 1, oxide powders ofvarious rare earth elements having average grain sizes of 0.9-1 μm asshown in Tables 3-4, MgO powder having an average grain size of 0.5 μm,Al₂O₃ powder AlN powder having an average grain size of 1 μm, andpowders of various compounds having average grain sizes of 0.4-0.5 μmwith Si₃N₄ (silicon nitride) material powder used in Example 1,

[0097] After thus obtained respective material mixtures were subjectedto the molding/degreasing operations under the same conditions as inExample 1, the compacts were subjected to the sintering operation andHIP treatment under the conditions shown in Tables 3-4, thereby toprepare a number of silicon nitride wear resistant members of Examples4-35.

COMPARATIVE EXAMPLES 5-14

[0098] Material mixtures of Comparative Examples 5-14 were respectivelyprepared as indicated in Tables 34. More specifically, an excessivelysmall amount or an excessive amount of various additives such as oxideof rare earth element such as Y₂O₃ or the like, MgAl₂O₄ spinel, SiC orthe like were added thereby to prepare the material mixtures for therespective Comparative Examples.

[0099] After thus obtained respective material mixtures were subjectedto the molding/degreasing operations under the same conditions as inExample 4, the compacts were subjected to the sintering operation andHIP treatment under the conditions shown in Tables 34, thereby tomanufacture a number of silicon nitride wear resistant members ofComparative Examples 5-14.

[0100] With respect to thus prepared plate-shaped silicon nitride wearresistant members of Examples and Comparative Examples, porosity,maximum width and average width of the aggregated segregations formed inthe grain boundary phase (liquid phase), three-point bending strength atroom temperature, fracture toughness and rolling life of the circularplates were measured under the same conditions as in Example 1. Themeasured results are shown in Tables 3-4 hereunder. TABLE 3 MaterialComposition (mass %) Sintering Condition HIP Condition Rare EarthMgAl₂O₄ Temp. × Time × Press. Temp. × Time × Press. Sample Si₃N₄ OxideSpinel SiC Other (° C.) × (hr) × (MPa) (° C.) × (hr) × (MPa) Example  489 Y₂O₃ 5 5 1 1550 × 6 × 0.01 1500 × 1 × 100  5 85 Y₂O₃ 5 5 5 1550 × 6 ×0.01 1500 × 1 × 100  6 80 Y₂O₃ 5 5 10 1550 × 6 × 0.01 1500 × 1 × 100  790 Y₂O₃ 2 5 3 1600 × 6 × 0.01 1500 × 1 × 100  8 82 Y₂O₃ 10 5 3 1600 × 6× 0.01 1500 × 1 × 100  9 90 Y₂O₃ 5 2 3 1600 × 6 × 0.01 1500 × 1 × 100 1083 Y₂O₃ 5 7 5 1550 × 6 × 0.01 1500 × 1 × 100 11 84 Y₂O₃ 5 5 5 TiO₂ 11550 × 6 × 0.01 1500 × 1 × 100 12 83 Y₂O₃ 5 5 5 TiO₂ 2 1550 × 6 × 0.011500 × 1 × 100 13 80 Y₂O₃ 5 5 5 TiO₂ 5 1550 × 6 × 0.01 1500 × 1 × 100 1483 Y₂O₃ 5 5 5 ZrO₂ 2 1550 × 6 × 0.01 1500 × 1 × 100 15 83 Y₂O₃ 5 5 5HfO₂ 2 1550 × 6 × 0.01 1500 × 1 × 100 16 83 Y₂O₃ 5 5 5 WC 2 1550 × 6 ×0.01 1500 × 1 × 100 17 83 Y₂O₃ 5 5 5 MO₂C 2 1550 × 6 × 0.01 1500 × 1 ×100 18 83 Y₂O₃ 5 5 5 Ta₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 19 83 Y₂O₃ 55 5 Nb₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 20 83 Y₂O₃ 5 5 5 Cr₂O3 2 1550× 6 × 0.01 1500 × 1 × 100 21 83 Y₂O₃ 5 5 5 TiO₂ 1 1550 × 6 × 0.01 1500 ×1 × 100 ZrO₂ 1 22 83 Y₂O₃ 5 5 5 TiO₂ 1 1600 × 6 × 0.01 None ZrO₂ 1 23 85CeO₂ 5 5 5 1550 × 6 × 0.01 1500 × 1 × 100 24 83 Er₂O₃ 7 5 5 1550 × 6 ×0.01 1500 × 1 × 100 25 83 Nd₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 2683 Sm₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 27 83 Ho₂O₃ 7 5 5 1550 × 6× 0.01 1500 × 1 × 100 28 83 Yb₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 10029 83 Dy₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 Comparative Example  589 Y₂O₃ 1 5 5 1550 × 6 × 0.01 1500 × 1 × 100  6 75 Y₂O₃ 15 5 5 1550 × 6× 0.01 1500 × 1 × 100  7 89 Y₂O₃ 5 1 5 1550 × 6 × 0.01 1500 × 1 × 100  881 Y₂O₃ 5 9 5 1550 × 6 × 0.01 1500 × 1 × 100  9 75 Y₂O₃ 5 5 15 1550 × 6× 0.01 1500 × 1 × 100 10 78 Y₂O₃ 5 5 5 TiO₂ 7 1550 × 6 × 0.01 1500 × 1 ×100 Width of Aggregated Rolling Segregation in Liquid Three-Point LifePhase(Grain Boundary Bending Fracture of Circular Porosity Phase) (μm)Strength Toughness Plate Sample (%) Average Maximum (MPa) (MPa ·m^(1/2)) (rotations) Example  4 0.01 1 2 1050 6.8   6 × 10⁷  5 0.02 0.61.5 1100 6.9 >1 × 10⁸  6 0.02 0.4 1 1035 6.5 >1 × 10⁸  7 0.08 1.5 2.51000 6.5 >1 × 10⁸  8 0.02 1.5 2.5 980 6.6 >1 × 10⁸  9 0.08 0.5 1 10006.8 >1 × 10⁸ 10 0.02 1 2 1050 6.6 >1 × 10⁸ 11 0.01 0.5 1 1125 6.8 >1 ×10⁸ 12 0.01 0.4 1 1110 6.8 >1 × 10⁸ 13 0.01 0.4 1 1000 6.5 >1 × 10⁸ 140.01 0.6 1.5 1100 6.8 >1 × 10⁸ 15 0.01 0.5 1 1090 6.9 >1 × 10⁸ 16 0.010.5 1 1040 6.7 >1 × 10⁸ 17 0.01 0.5 1 1085 6.8 >1 × 10⁸ 18 0.01 0.5 11040 6.8 >1 × 10⁸ 19 0.01 0.5 1 1040 6.8 >1 × 10⁸ 20 0.01 0.5 1 10006.7 >1 × 10⁸ 21 0.01 0.5 1 1130 7.0 >1 × 10⁸ 22 0.2 0.3 1 1050 6.9   5 ×10⁷ 23 0.01 0.6 1.5 1020 6.8 >1 × 10⁸ 24 0.01 0.5 1.5 1100 7.0 >1 × 10⁸25 0.01 0.5 1 1070 6.5 >1 × 10⁸ 26 0.01 0.5 1.5 1050 6.5 >1 × 10⁸ 270.01 0.5 1 1080 6.7 >1 × 10⁸ 28 0.01 0.5 1 1010 6.7 >1 × 10⁸ 29 0.01 0.51.5 1050 6.8 >1 × 10⁸ Comparative Example  5 0.2 0.7 1.5 890 5.9   6 ×10⁶  6 0.05 2 4 880 5.8   1 × 10⁶  7 1.5 0.5 1 890 6.4   2 × 10⁶  8 0.011 2 900 6.3   8 × 10⁶  9 1.5 0.3 1 850 5.8   2 × 10⁶ 10 0.02 0.7 1.5 8906.3   5 × 10⁶

[0101] TABLE 4 Plate-Shaped Wear Resistant Member Material Composition(mass %) Sintering Condition HIP Condition Rare Earth Temp. × Time ×Press. Temp. × Time × Press. Sample Si₃N₄ Oxide Al₂O₃ MgO SiC Other (°C.) × (hr) × (MPa) (° C.) × (hr) × (MPa) Example 30 86 Y₂O₃ 5 2 2 5 1550× 6 × 0.01 1500 × 1 × 100 31 84 Y₂O₃ 5 5 1 5 1550 × 6 × 0.01 1500 × 1 ×100 32 84 Y₂O₃ 5 4 2 5 1550 × 6 × 0.01 1500 × 1 × 100 33 86 Y₂O₃ 5 4 2 31550 × 6 × 0.01 1500 × 1 × 100 34 83 Y₂O₃ 5 3.5 1.5 5 TiO₂ 1 1550 × 6 ×0.01 1500 × 1 × 100 ZrO₂ 1 35 83 Y₂O₃ 5 3.5 1.5 5 TiO₂ 1 1550 × 6 × 0.01None ZrO₂ 1 Comparative Example 11 88 Y₂O₃ 5 1 1 5 1550 × 6 × 0.01 1500× 1 × 100 12 81 Y₂O₃ 5 7 2 5 1550 × 6 × 0.01 1500 × 1 × 100 13 86.5 Y₂O₃5 3 0.5 5 1550 × 6 × 0.01 1500 × 1 × 100 14 81 Y₂O₃ 5 5 4 5 1550 × 6 ×0.01 1500 × 1 × 100 Width of Aggregated Rolling Segregation in LiquidThree-Point Life Phase(Grain Boundary Bending Fracture of CircularPorosity Phase) (μm) Strength Toughness Plate Sample (%) Average Maximum(MPa) (MPa · m^(1/2)) (rotations) Example 30 0.01 0.3 1 1100 6.9 >1 ×10⁸ 31 0.01 0.5 1.5 1070 6.6 >1 × 10⁸ 32 0.01 0.6 1.5 1080 6.7 >1 × 10⁸33 0.01 0.8 1.5 1120 7.0 >1 × 10⁸ 34 0.01 0.4 1 1110 6.9 >1 × 10⁸ 35 0.30.3 1 1000 6.8   4 × 10⁷ Comparative Example 1.5 0.5 1 880 6.2   2 × 10⁶0.01 0.8 2 900 6.2   7 × 10⁶ 13 1.2 0.5 1 850 6.5   4 × 10⁶ 14 0.01 1.83.5 890 6.4   5 × 10⁶

[0102] As is clear from the results shown in Tables 3-4, in therespective silicon nitride wear resistant members of Examples each ofwhich was manufactured in such a manner that the material powder mixturecontaining specified additives was molded and sintered, followed bybeing subjected to HIP treatment as occasion demanded after completionof the sintering process, the generation of the pore was effectivelysuppressed thereby to enable the width of the aggregated segregationsformed in the grain boundary phase to be extremely small. Therefore,there could be obtained the silicon nitride wear resistant membershaving good strength characteristics and an excellent durability suchthat the rolling life exceeded 108 for most cases of Examples.

[0103] On the other hand, in the silicon nitride sintered bodies asshown in Comparative Examples 5-14 in which the amount of additives suchas rare earth components were set to outside the range specified in thepresent invention, even if the sintering operation or the HIP treatmentafter the sintering operation were sufficiently performed, the rollinglife of the wear resistant members were lowered. Further, it wasconfirmed that the sintered bodies of Comparative Examples could notsatisfy at least one of the required characteristics such as porosity,width of the aggregated segregation, three-point bending strength,fracture toughness value or the like that were specified in the presentinvention.

[0104] Next, preferred embodiments of the wear resistant members of theabove Examples 4-35 and Comparative Examples 5-14 applied to rollingballs of a bearing member will be explained more concretely on the basisof the following Examples and Comparative Examples.

EXAMPLES 4B-35B and COMPARATIVE EXAMPLES 5B-14B

[0105] Each of the blended granulated powders as prepared in Examples4-35 and Comparative Examples 5-14 was packed in the molding die andpressed thereby to prepare spherical primary molded bodies. Then, eachof the primary molded bodies was subjected to a rubber pressingtreatment at a pressure of 100 MPa, thereby to respectively preparespherically molded bodies as samples each having a diameter of 11 mm formeasuring crush strength and rolling fatigue life.

[0106] Next, after the respective spherically molded bodies weresubjected to the degreasing treatment under the same conditions as inExample 1, the degreased bodies were treated under the conditions of thesintering operations and HIP treatments as shown in tables 5-6. Further,thus obtained sintered bodies were subjected to grinding work so as toprovide a ball-shape having a diameter of 9.52 mm and a surfaceroughness of 0.01 μm-Ra thereby to prepare the respective rolling ballsfor a bearing as wear resistant members of Examples 4B-35B andComparative Examples 5B-14B.

[0107] In this connection, the above surface roughness was measured asan arithmetic average surface roughness (Ra) obtained by scanning thesurface on equator of the ball by means of a profilometer-type surfaceroughness measuring device.

[0108] With respect to thus prepared rolling balls as the wear resistantmembers of the respective Examples and Comparative Examples, porosity,width of the aggregated segregation, crush strength, fracture toughnessvalue and rolling fatigue life were measured as the same manner as inExample 1B. The measured results are shown in Tables 5-6 hereunder.TABLE 5 Rolling Ball-Shaped Wear Resistant Member Material Composition(mass %) Sintering Condition HIP Condition Rare Earth MgAl₂O₄ Temp. ×Time × Press. Temp. × Time × Press. Sample Si₃N₄ Oxide Spinel SiC Other(° C.) × (hr) × (MPa) (° C.) × (hr) × (MPa) Example  4B 89 Y₂O₃ 5 5 11550 × 6 × 0.01 1500 × 1 × 100  5B 85 Y₂O₃ 5 5 5 1550 × 6 × 0.01 1500 ×1 × 100  6B 80 Y₂O₃ 5 5 10 1550 × 6 × 0.01 1500 × 1 × 100  7B 90 Y₂O₃ 25 3 1600 × 6 × 0.01 1500 × 1 × 100  8B 82 Y₂O₃ 10 5 3 1600 × 6 × 0.011500 × 1 × 100  9B 90 Y₂O₃ 5 2 3 1600 × 6 × 0.01 1500 × 1 × 100 10B 83Y₂O₃ 5 7 5 1550 × 6 × 0.01 1500 × 1 × 100 11B 84 Y₂O₃ 5 5 5 TiO₂ 1 1550× 6 × 0.01 1500 × 1 × 100 12B 83 Y₂O₃ 5 5 5 TiO₂ 2 1550 × 6 × 0.01 1500× 1 × 100 13B 80 Y₂O₃ 5 5 5 TiO₂ 5 1550 × 6 × 0.01 1500 × 1 × 100 14B 83Y₂O₃ 5 5 5 ZrO₂ 2 1550 × 6 × 0.01 1500 × 1 × 100 15B 83 Y₂O₃ 5 5 5 HfO₂2 1550 × 6 × 0.01 1500 × 1 × 100 16B 83 Y₂O₃ 5 5 5 WC 2 1550 × 6 × 0.011500 × 1 × 100 17B 83 Y₂O₃ 5 5 5 MO₂C 2 1550 × 6 × 0.01 1500 × 1 × 10018B 83 Y₂O₃ 5 5 5 Ta₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 19B 83 Y₂O₃ 5 55 Nb₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 20B 83 Y₂O₃ 5 5 5 Cr₂O3 2 1550× 6 × 0.01 1500 × 1 × 100 21B 83 Y₂O₃ 5 5 5 TiO₂ 1 1550 × 6 × 0.01 1500× 1 × 100 ZrO₂ 1 22B 83 Y₂O₃ 5 5 5 TiO₂ 1 1600 × 6 × 0.01 None ZrO₂ 123B 85 CeO₂ 5 5 5 1550 × 6 × 0.01 1500 × 1 × 100 24B 83 Er₂O₃ 7 5 5 1550× 6 × 0.01 1500 × 1 × 100 25B 83 Nd₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 ×100 26B 83 Sm₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 27B 83 Ho₂O₃ 7 5 51550 × 6 × 0.01 1500 × 1 × 100 28B 83 Yb₂O₃ 7 5 5 1550 × 6 × 0.01 1500 ×1 × 100 29B 83 Dy₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 ComparativeExample  5B 89 Y₂O₃ 1 5 5 1550 × 6 × 0.01 1500 × 1 × 100  6B 75 Y₂O₃ 155 5 1550 × 6 × 0.01 1500 × 1 × 100  7B 89 Y₂O₃ 5 1 5 1550 × 6 × 0.011500 × 1 × 100  8B 81 Y₂O₃ 5 9 5 1550 × 6 × 0.01 1500 × 1 × 100  9B 75Y₂O₃ 5 5 15 1550 × 6 × 0.01 1500 × 1 × 100 10B 78 Y₂O₃ 5 5 5 TiO₂ 7 1550× 6 × 0.01 1500 × 1 × 100 Width of Aggregated Segregation in LiquidRolling Phase(Grain Boundary Crush Fracture Fatigue Porosity Phase) (μm)Strength Toughness Life of Sample (%) Average Maximum (MPa) (MPa ·m^(1/2)) Ball (hr) Example  4B 0.01 1 2 235 6.8 >400  5B 0.02 0.6 1.5275 6.9 >400  6B 0.02 0.4 1 230 6.5 >400  7B 0.08 1.5 2.5 220 6.5 >400 8B 0.02 1.5 2.5 215 6.6 >400  9B 0.08 0.5 1 230 6.8 >400 10B 0.02 1 2250 6.6 >400 11B 0.01 0.5 1 285 6.8 >400 12B 0.01 0.4 1 270 6.8 >400 13B0.01 0.4 1 225 6.5 >400 14B 0.01 0.6 1.5 270 6.8 >400 15B 0.01 0.5 1 2706.9 >400 16B 0.01 0.5 1 240 6.7 >400 17B 0.01 0.5 1 260 6.8 >400 18B0.01 0.5 1 245 6.8 >400 19B 0.01 0.5 1 240 6.8 20B 0.01 0.5 1 2106.7 >400 21B 0.01 0.5 1 285 7.0 >400 22B 0.2 0.3 1 245 6.9 >400 23B 0.010.6 1.5 230 6.8 >400 24B 0.01 0.5 1.5 270 7.0 >400 25B 0.01 0.5 1 2556.5 >400 26B 0.01 0.5 1.5 240 6.5 >400 27B 0.01 0.5 1 260 6.7 >400 28B0.01 0.5 1 225 6.7 >400 29B 0.01 0.5 1.5 235 6.8 >400 ComparativeExample  5B 0.2 0.7 1.5 195 5.9 295  6B 0.05 2 4 190 5.8 200  7B 1.5 0.51 200 6.4 225  8B 0.01 1 2 200 6.3 330  9B 1.5 0.3 1 175 5.8 230 10B0.02 0.7 1.5 195 6.3 260

[0109] TABLE 6 Rolling Ball-Shaped Wear Resistant Member MaterialComposition (mass %) Sintering Condition HIP Condition Rare Earth Temp.× Time × Press. Temp. × Time × Press. Sample Si₃N₄ Oxide Al₂O₃ MgO SiCOther (° C.) × (hr) × (MPa) (° C.) × (hr) × (MPa) Example 30B 86 Y₂O₃ 52 2 5 1550 × 6 × 0.01 1500 × 1 × 100 31B 84 Y₂O₃ 5 5 1 5 1550 × 6 × 0.011500 × 1 × 100 32B 84 Y₂O₃ 5 4 2 5 1550 × 6 × 0.01 1500 × 1 × 100 33B 86Y₂O₃ 5 4 2 3 1550 × 6 × 0.01 1500 × 1 × 100 34B 83 Y₂O₃ 5 3.5 1.5 5 TiO₂1 1550 × 6 × 0.01 1500 × 1 × 100 ZrO₂ 1 35B 83 Y₂O₃ 5 3.5 1.5 5 TiO₂ 11550 × 6 × 0.01 None ZrO₂ 1 Comparative Example 11B 88 Y₂O₃ 5 1 1 5 1550× 6 × 0.01 1500 × 1 × 100 12B 81 Y₂O₃ 5 7 2 5 1550 × 6 × 0.01 1500 × 1 ×100 13B 86.5 Y₂O₃ 5 3 0.5 5 1550 × 6 × 0.01 1500 × 1 × 100 14B 81 Y₂O₃ 55 4 5 1550 × 6 × 0.01 1500 × 1 × 100 Width of Aggregated Segregation inLiquid Rolling Phase(Grain Boundary Crush Fracture Fatigue PorosityPhase) (μm) Strength Toughness Life of Sample (%) Average Maximum (MPa)(MPa · m^(1/2)) Ball (hr) Example 30B 0.01 0.3 1 270 6.9 >400 31B 0.010.5 1.5 260 6.6 >400 32B 0.01 0.6 1.5 260 6.7 >400 33B 0.01 0.8 1.5 2807.0 >400 34B 0.01 0.4 1 270 6.9 >400 35B 0.3 0.3 1 225 6.8 >400Comparative Example 11B 1.5 0.5 1 190 6.2 225 12B 0.01 0.8 2 205 6.2 28013B 1.2 0.5 1 180 6.5 245 14B 0.01 1.8 3.5 195 6.4 255

[0110] As is clear from the results shown in Tables 5-6, in therespective silicon nitride rolling balls of Examples each of which wasmanufactured in such a manner that the material powder mixturecontaining specified amounts of various additives such as rare earthelement, MgAl₂O₄ spinel, SiC or the like was molded and sintered,followed by being subjected to HIP treatment as occasion demanded aftercompletion of the sintering process, the generation of the pore waseffectively suppressed thereby to decrease the size of the aggregatedsegregation in the grain boundary to be extremely small. Therefore,there could be obtained the silicon nitride rolling balls each having ahigh crush strength and an excellent durability such that the rollingfatigue life exceeded 400 hours.

[0111] On the other hand, in the silicon nitride sintered bodies asshown in Comparative Examples 5B-14B in which the amount of additivessuch as rare earth components were set to outside the range specified inthe present invention, even if the sintering operation and the HIPtreatment were sufficiently performed, the rolling fatigue life of therolling balls was lowered, and it was confirmed that the sintered bodiesof Comparative Examples could not satisfy at least one of the requiredcharacteristics such as porosity, width of the aggregated segregation,three-point bending strength or the like that were specified in thepresent invention.

Industrial Applicability

[0112] As described above, according to the silicon nitride wearresistant member and the method of manufacturing the member of thepresent invention, the material mixture is prepared by adding thepredetermined amounts of the rare earth element, MgAl₂O₄ spinel or themixture of magnesium oxide and aluminum oxide, silicon carbide, andcompound of Ti, Zr, Hf or the like to the silicon nitride materialpowder, so that the sintering property is greatly improved. Therefore,even if the molded compact is sintered at a low temperature of 1600° C.or lower, there can be obtained a silicon nitride wear resistant memberhaving an excellent wear resistance, a high density and a highmechanical strength that are equal to or higher than those ofconventional silicon nitride sintered body. In particular, the siliconnitride wear resistant member is suitable for a material constituting arolling bearing member in view of its excellent rolling lifecharacteristics.

[0113] Further, the generation of pores is suppressed and a maximum size(diameter) of pore can be minimized, so that there can be obtained awear resistant member excellent in rolling life characteristics anddurability. Accordingly, when a bearing device is prepared by using thiswear resistant member as rolling bearing member, good sliding/rollingcharacteristics can be maintained for a long time of period, and therecan be provided a rotation machine having excellent operationalreliability and durability.

1. A wear resistant member composed of silicon nitride sintered body containing 2-10 mass % of rare earth element in terms of oxide thereof as sintering agent, 2-7 mass % of MgAl₂O₄ spinel, 1-10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of said silicon nitride sintered body is 1 vol. % or less, a three-point bending strength is 900 MPa or more, a fracture toughness is 6.3 MPa·m^(1/2) or more, and a maximum width of aggregated segregation existing in grain boundary phase of the silicon nitride sintered body is 5 μm or less.
 2. A wear resistant member composed of silicon nitride sintered body containing 2-10 mass % of rare earth element in terms of oxide thereof as sintering agent, 2-7 mass % of MgAl₂O₄ spinel, 1-10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of said silicon nitride sintered body is 1 vol. % or less, a three-point bending strength is 900 MPa or more, a fracture toughness is 6.3 MPa·m^(1/2) or more, and an average width of aggregated segregation existing in grain boundary phase of the silicon nitride sintered body is 2 at m or less.
 3. A wear resistant member composed of silicon nitride sintered body containing 2-10 mass % of rare earth element in terms of oxide thereof as sintering agent, 1-2 mass % of magnesium oxide, 2-5 mass % of aluminum oxide, 1-10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of said silicon nitride sintered body is 1 vol. % or less, a three-point bending strength is 900 MPa or more, and a fracture toughness is 6.3 MPa·m^(1/2) or more, and a maximum width of aggregated segregation existing in grain boundary phase of the silicon nitride sintered body is 5 μm or less.
 4. A wear resistant member composed of silicon nitride sintered body containing 2-10 mass % of rare earth element in terms of oxide thereof as sintering agent, 1-2 mass % of magnesium oxide, 2-5 mass % of aluminum oxide, 1-10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of said silicon nitride sintered body is 1 vol. % or less, a three-point bending strength is 900 MPa or more, and a fracture toughness is 6.3 MPa·m^(1/2) or more, and an average width of aggregated segregation existing in grain boundary phase of the silicon nitride sintered body is 2 μm or less.
 5. (canceled)
 6. (canceled)
 7. A method of manufacturing a wear resistant member composed of silicon nitride sintered body comprising the steps of: preparing a material mixture by adding 2-10 mass % of a rare earth element in terms of the amount of an oxide thereof, 2-7 mass % of MgAl₂O₄ spinel, 1-10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, to a silicon nitride powder containing 1.5 mass % or less of oxygen and 90 mass % or more of α-phase type silicon nitride and having an average grain size of 1 μm or less; molding said material mixture to form a compact; degreasing said compact; and sintering the compact in non-oxidizing atmosphere at a temperature of 1,600° C. or lower thereby to form a wear resistant member composed of silicon nitride sintered body.
 8. A method of manufacturing a wear resistant member composed of silicon nitride sintered body comprising the steps of: preparing a material mixture by adding 2-10 mass % of a rare earth element in terms of the amount of an oxide thereof, 1-2 mass % of magnesium oxide, 2-5 mass % of aluminum oxide, 1-10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, to a silicon nitride powder containing 1.5 mass % or less of oxygen and 90 mass % or more of α-phase type silicon nitride and having an average grain size of 1 μm or less; molding said material mixture to form a compact; degreasing said compact; and sintering the compact in non-oxidizing atmosphere at a temperature of 1,600° C. or lower thereby to form a wear resistant member composed of silicon nitride sintered body.
 9. The method of manufacturing the wear resistant member composed of silicon nitride sintered body according to claim 7 or 8, wherein said method further comprising the step of: conducting a hot isostatic pressing (HIP) treatment to said silicon nitride sintered body in non-oxidizing atmosphere of 30 MPa or more at a temperature of 1,600° C. or lower after completion of the sintering step. 